徹底的てっていてきに vs. 完全かんぜんに: Which is A Much Stronger Meaning?

When you want to say “completely” or “thoroughly” in Japanese, two adverbs come to mind: 徹底的に (tetteitekini) and 完全に (kanzenni). Both can translate as “completely” in English, but their nuances are quite different — one expresses deep, determined effort, the other expresses a state of totality or perfection.

Yuka

Rei, I want to say “I completely cleaned my room.” Should I use 徹底的に or 完全に?

Rei

Both could work, but they feel different! 徹底的に emphasizes thoroughness and determined effort — like you left no corner untouched. 完全に emphasizes completeness or totality — the state of being 100% done. For cleaning, 徹底的に掃除した sounds very thorough and intentional.

At a Glance: 徹底的に vs 完全に

徹底的に (Tetteitekini) — Thoroughly, Without Exception

徹底的に derives from 徹底（てってい）, meaning “thoroughness” or “exhaustiveness.” The に turns it into an adverb modifying verbs. It emphasizes that something is done completely and rigorously — nothing is skipped, every aspect is addressed. You’ll often see it paired with actions like cleaning, investigating, studying, or preparing. It carries a sense of determination and precision.

▶ 徹底的に調べた結果、問題は見つからなかった。
Tetteitekini shirabeta kekka, mondai wa mitsuka ranakatta.
After thoroughly investigating, no problems were found.

▶ 試験前に徹底的に復習した。
Shiken mae ni tetteitekini fukushuu shita.
I thoroughly reviewed everything before the exam.

▶ ルールを徹底的に守ることが大切だ。
Ruuru wo tetteitekini mamoru koto ga taisetsu da.
It is important to follow the rules thoroughly.

Yuka

So 徹底的に sounds very intense and determined — like you went all-in?

Rei

Exactly! 徹底的に has an energy of relentless effort. If someone says 徹底的に準備した, you imagine them preparing intensively and leaving nothing to chance.

完全に (Kanzenni) — Completely, 100%

完全に comes from 完全（かんぜん）, meaning “perfect” or “complete.” It describes something that is fully done, entirely the case, or in a state of totality — with no missing parts. Unlike 徹底的に, it doesn’t emphasize the effort or process; it emphasizes the result or state. It can describe facts, states, and conditions — not just actions. You’ll hear 完全に in news articles, scientific statements, and everyday conversation.

▶ 工事は完全に終わった。
Kouji wa kanzenni owatta.
The construction is completely finished.

▶ 彼の言っていることは完全に間違いだ。
Kare no itte iru koto wa kanzenni machigai da.
What he’s saying is completely wrong.

▶ 完全に治った！
Kanzenni naotta!
I’m completely recovered!

Yuka

I see — 完全に is about the state being total or 100%, not about how hard you worked?

Rei

Perfect! 完全に is neutral about effort. It just says “this is total/complete/100%.” 徹底的に says “this was done with exhaustive effort.”

Common Mistakes

Side-by-Side Comparison

Decision Flowchart

Are you modifying a verb or describing a situation?
         |
         v
Does it describe a state or condition being 100% true?
  YES → 完全に ✅ (e.g., 完全に間違い, 完全に終わった)
  NO  ↓
Does it describe an action done with thorough, determined effort?
  YES → 徹底的に ✅ (e.g., 徹底的に調べる, 徹底的に準備する)
Tip: 完全に can also modify adjectives (完全に正しい), 徹底的に cannot.

Quick Quiz

Yuka

Let me test myself on these adverbs!

Rei

Choose 徹底的に or 完全に for each sentence.

Q1. “I completely forgot about the meeting.”
会議のことを___忘れた → 完全に ✅

Q2. “She studied thoroughly for the exam.”
試験のために___勉強した → 徹底的に ✅

Q3. “The rumor is completely false.”
その噂は___嘘だ → 完全に ✅

Q4. “They thoroughly inspected the building for defects.”
建物の欠陥を___調べた → 徹底的に ✅

Q5. “The project is now completely finished.”
プロジェクトは___終了した → 完全に ✅

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